In a gas turbine engine, inlet air is continuously compressed, mixed with liquid fuel in an inflammable proportion, and then contacted with an ignition source to ignite the mixture which will then continue to burn. The heat energy thus released then flows in the combustion gases to a turbine where it is expanded and converted to rotary energy for driving equipment such an electrical generator. The combustion gases are then exhausted to atmosphere after giving up some of the remaining heat to the incoming air provided from the compressor.
As is well known, gas turbine engines typically include a rotor and a turbine wheel rotatable about a generally horizontal axis. Not infrequently, an annular combustor surrounds this horizontal axis and is provided with a plurality of angularly spaced liquid fuel injectors whereby liquid fuel is injected into the combustor to be ignited and burned and the combustion products ultimately directed at the turbine wheel to spin the turbine wheel. At a location that is usually external of the combustor, a ring-like manifold is utilized as a liquid fuel manifold to interconnect the various annularly spaced liquid fuel injectors.
Because the rotational axis of the compressor and turbine wheel is horizontal, this ring-like manifold would normally be in a vertical plane. This in turn means that the pressure acting on the liquid fuel at the lowermost liquid fuel injector will be greater than the pressure acting on the liquid fuel at the highest injector. The pressure difference is a consequence of gravity on the vertical column of liquid fuel in the manifold and thus is generally referred to as "manifold head".
While in a larger gas turbine this may not represent a significant problem, in a small gas turbine with nominally small liquid fuel flows, substantial nonuniformity in liquid fuel injection may be produced. This in turn can lead to the development of hot spots within the small gas turbine engine combustor which can shorten its life as well as reduce operating efficiencies because of poor localized combustion. Achieving uniform turbine inlet temperature distribution minimizes hot spots and cold spots to maximize efficiency of operation as well as to prolong the life of the turbine parts exposed to the hot combustion gases.
While a simple solution might be to provide a large number of liquid fuel injectors to insure that the liquid fuel is uniformly distributed to the combustion air, the number of liquid fuel injectors not only increase costs, but also means that each individual liquid fuel injector would be smaller when the overall liquid fuel consumption remains the same.
Also, while each liquid fuel injector can theoretically be provided with an individual orifice, this requires an increase in liquid fuel pressure in order to deliver liquid fuel past the orifice into the combustion chamber. As a consequence, in order to have substantially uniform liquid fuel injection at all injector locations, the manifold head pressure at the lowermost liquid fuel injector would be relatively small compared to the pressure applied to the liquid fuel at all other orifices. In order to increase this pressure drop at each liquid fuel injector location, the orifices must be made to be relatively small. As a consequence, these small orifices would be prone to clogging. Once an orifice is clogged and the corresponding liquid fuel orifice is blocked, the problem of hot spots returns.
The design of combustion systems for small gas turbines is hardly a simple scale down of designs that are operative in large gas turbine engines. Regardless of combustor size, there is a minimum residence time for liquid fuel and air within the combustor necessary to effect sufficiently complete combustion to generate the gases to drive a turbine wheel. Given the dynamics of gas flow in and out of a combustor to a turbine wheel, it should be readily apparent that as the size of the combustor is decreased, conventional techniques would only be starting the combustion process, if it occurred at all, as the air and liquid fuel mixture was exiting the combustor outlet.
Moreover, in small combustors, which necessarily are provided with small liquid fuel injectors and consequently have relatively small liquid fuel flow at each injector, it is difficult to provide the needed fine liquid fuel atomization with conventional techniques. This is primarily due to the fact that the small scale effects increased viscous losses resulting in a deterioration in liquid fuel atomization at the injector. In addition, the small liquid fuel metering orifices associated with such small liquid fuel injectors tend to promote premature liquid fuel spray deterioration due to orifice fouling which in turn can cause early engine failure due to gas temperature maldistributions. Conventional liquid fuel injector design techniques are already ordinarily complex and costly. When, however, they are employed to reduced scale design for use in small combustors, the complexity and cost becomes prohibitive.
Recognizing these difficulties, in recent years there has been a definite trend towards combustor systems in which the path of travel for the liquid fuel and air in the flame zone, as well as the products of combustion, are in the circumferential direction rather than in the axial direction as in a conventional combustion system. These annular combustors employ a technique called "sidewinding" to minimize the axial flow components of liquid fuel, air and products of combustion. This arrangement maximizes the time available for combustion within a given small volume and also permits a significant reduction in the number of liquid fuel injectors without the resultant undesirable high turbine inlet temperature maldistributions as would be obtained using conventional design techniques if the number of liquid fuel injectors is reduced. Maximizing the time available for mixing and combustion while minimizing the number of liquid fuel injectors is most advantageous from cost and efficiency standpoints, particularly when accomplished in small gas turbines.
In recent annular combustors operating on the sidewinder technique, it is typical to have a plurality of air blast and/or air assist tubes circumferentially spaced about the combustor and normally located in the radially outer wall thereof. While air blast and air assist are somewhat similar, air blast generally has a higher velocity and is hotter than air assist. One end of each tube is open to the interior of the combustor while the opposite end is opened to the space between the radially outer wall of the combustor and the outer combustor case. As is known, this space is typically charged with compressed air from the compressor associated with the gas turbine engine. These tubes are directed tangentially into the annular combustion space of the combustor.
For liquid fuel injection purposes, liquid fuel injector tubes have typically been mounted within the air assist tubes and as a consequence liquid fuel atomization of liquid fuel injected from the tubes may be achieved as the liquid fuel is injected toward the combustion space in an associated air assist tube as the air passing though the air assist tube provides air atomization. As smaller and smaller combustors are designed, however, the diameter of the air assist tubes becomes commensurately reduced to the point where it is difficult to place the liquid fuel injector tubes inside the air assist tube.
Also, since the liquid fuel injector orifices or outlets are within the combustor, they are exposed to substantial heat. During normal operations, this does not present a problem since the flow of liquid fuel through the liquid fuel injector provides a cooling effect. Further, the propagation of combustion along with the flow of air serves to prevent undesirable overheating of the liquid fuel injectors. Once, however, operation ceases, neither liquid fuel nor air flows through the liquid fuel injector. Consequently, residual heat in the combustor area will cause elevation of the temperature of the liquid fuel injectors. In terms of the materials of which the liquid fuel injectors are constructed, this raising in temperature upon cessation of operation does not present a problem. The presence, however, of residual liquid fuel in the liquid fuel injector at such time will frequently cause a coking problem. Being carbonaceous in nature, such liquid fuel, upon being heated will begin to undergo a destructive distillation reaction and a coke-like and/or tarry residue will remain. Such a residue will quickly clog the liquid fuel injector and will result in improper operation during subsequent start-up.